PROJECT SUMMARY Colorectal cancer (CRC), the third leading cause of cancer deaths in the United States, is a challenging disease involving complex interplay among immune cells, cytokine-mediated pathways and gut microbiota ? leading to inflammation, disruption of the epithelial tissue barrier, acute colitis and CRC development. Thus, mechanistic studies are urgently needed to clarify the interactions and consequences of these key variables, enabling the identification of novel targets for therapeutic intervention. In the course of studies funded by our previous award (R01 CA101795), we identified TAK1 (TGF-beta-activated kinase 1) as a negative regulator of NF-?B signaling in neutrophils and found that its specific deletion in the myeloid lineage (Tak1?M/?M) leads to increased proinflammatory cytokines (IL-1?, IL-6, and TNF-?) and greater sensitivity to LPS-induced septic shock. Unexpectedly, Tak1?M/?M mice were completely resistant to chemically induced acute colitis and CRC development. This phenotype was associated with a marked accumulation of Th17 and ILC3 cells in the intestinal lamina propria and alterations of the gut microbiota composition. Further studies showed significant suppression of chemokine expression in colon epithelial cells and of immune cell trafficking into colon tissues. These intriguing new findings led us to hypothesize that specific gut microbiota, Th17 and ILC3 cells and their cytokines (IL-17 and IL-22) play critical roles in regulating resistance to colitis and inflammation-related CRC through the suppression of chemokine expression by colon epithelial cells. Key predictions of this hypothesis will be tested in three specific research aims, all relying on the Tak1?M/?M model of suppressed colitis and CRC tumorigenesis. Aim 1 seeks to identify specific strains of microbiota that contribute to the accumulation of Th17 and ILC3 cells and thus to resistance to experimental colitis and CRC. Aim 2 will define the functions of Th17 and ILC3 cells (and of their cytokines) in promoting resistance to chemically induced colitis and CRC, while Aim 3 will test the hypothesis that Th17/ILC3-derived cytokines inhibit the expression of chemokines by colon epithelial cells, thus blocking subsequent immune cell infiltration of the epithelial layer and ultimately the induction of malignant changes in the colon. Upon successful completion of this work, we expect to have a much-improved understanding of the inflammatory processes that drive the development of ulcerative colitis and CRC, and thus of potential vulnerabilities in this network that could be targeted for the prevention or treatment of these disorders.